In the field of regenerative medicine, a variety of artificial biomaterials that can be used as substituents to damaged tissues or organs have been recently developed. Implants such as artificial bones or artificial bone fillers are used particularly for treatment of bone defects. Such implants, however, are required to have bioadaptability and bone inductivity in addition to mechanical properties similar to those of natural bones. That is, implants needs to be gradually resorbed after implantation in the body, involved into the bone regeneration cycle, and then substituted for the bones of the subject.
Bones of vertebrates are composed of hydroxyapatite and collagen. They forms a specific nanocomposite structure in natural bones characterized in that the c-axis of hydroxyapatite is oriented along the collagen fibers, and this structure imparts bone-specific mechanical properties. Composite biomaterials of hydroxyapatite and collagen having structures and compositions similar to those of natural bones are described in, for example, JP Patent Publication (Kokai) Nos. 7-101708 A (1995) and 11-199209 A (1999), and bone inductivity thereof has been observed to some extent.
Alginic acid is a polysaccharide contained in seaweed, which has been heretofore employed in a haemostatic drug or wound dressing. Concerning artificial bones, development of bone fillers comprising α-TCP in combination with alginate has been reported (Nagata, Shika Zairyou (Dental materials), vol. 16, No. 6, 1997, pp. 479-491). Moreover, alginic acid has been recently reported to help a repair of bones and/or cartilages (e.g., Fragonas et al., Biomaterials 21, 2000, pp. 795-801). Application of alginic acid to a composite of hydroxyapatite and collagen, however, had not yet been attempted. In particular, homogenous incorporation of alginic acid into the composite while maintaining its specific nanocomposite structure involves some difficulties.